In sewing machines, an upper thread is supplied to a needle from a thread supply source through a thread take-up lever. The thread take-up lever changes the amount of upper thread to be supplied to the needle from the thread supply source. A lower thread is retained in a hook. A needle bar that supports the needle, and the thread take-up lever are driven by an upper shaft, while the hook is driven by a lower shaft. The upper shaft and the lower shaft are coupled through a toothed belt, and thus the thread take-up lever and the needle, and, the hook are mutually operated. Sewing machines catch, through a tip of the hook, a thread loop formed by the upper thread when the needle rises after falling to the needle bottom dead center, and form a stitch by intertwining the upper thread and the lower thread with each other.
More specifically, the hook includes an inner hook that retains therein the lower thread, and an outer hook that catches the upper thread, and performs a rotation movement along with the rotation of the lower shaft. During cycles of forming stitches, through the up-and-down movement of the needle bar, the upper thread inserted in a needle hole passes completely through a cloth, and the thread loop of the upper thread formed below the cloth is caught by the outer hook. The outer hook rotates while catching the upper thread, and the upper thread passes through the inner hook so as to go around the inner hook in conjunction with the motion of the outer thread. According to this operation, the upper thread and the lower thread intersect with each other, and thus a stitch is formed.
When the upper thread goes around the inner hook, the upper thread temporarily needs a length by what corresponds to the outer circumference of the inner hook. This necessary length of the upper thread is called a hook/upper-thread necessary amount, and is supplied by the descending thread take-up lever that is a thread guide for the upper thread. The upper thread supplied by the operation of the thread take-up lever is called a thread-take-up-lever/upper-thread supply amount. It is preferable that, during a sewing operation, the thread-take-up-lever/upper-thread supply amount should be set to be always larger than the hook/upper-thread necessary amount, and thus the upper thread should be slightly loosened. This is because a thread breakage can be suppressed by the loosened upper thread, and stitches can be formed smoothly.
In addition, since sewing machines are provided with a swing mechanism that swings the needle bar in a direction intersecting the cloth feeding direction, the sewing machines are capable of forming zig-zag stitches. When this swing mechanism is controlled to adjust the swing level of the needle bar and the timing thereof, complex sewing, such as a whipstitch, a pattern stitch, or a letter stitch, is realized.
When complex sewing is performed, if the swing of the needle bar becomes large, the position of the needle bar frequently changes. Hence, relative positional relationship between the needle and the hook changes, and the timing at which the needle and the hook are relatively operated changes together with the change in the relative positional relationship. When the change in this timing exceeds the allowable range, it becomes difficult to form a stitch.
Hence, according to conventional sewing machines, the swing level of the needle bar is limited so as to set the change in relative operation of the needle and the hook within a range that enables a formation of a stitch. Alternatively, two idlers that form an idler unit are caused to contact the toothed belt which couples the upper shaft and the lower shaft (see, for example, JP 2008-264500 A). The idler unit is driven in conjunction with the swing of the needle bar in the case of, for example, zig-zag stitches to change the belt length of the toothed belt at the tensioned side, thereby controlling a needle/hook intersecting phase that is the timing at which the needle and the hook are relatively operated. This control corrects the gap in timing of the needle that is linked with the swing of the needle bar, and that of the hook so as to enable a formation of a stitch even if the swing width is wide.
When the needle bar swings in zig-zag stitches, the needle bar is moving from side to side while the outer hook is catching the upper thread. Hence, the upper thread inserted in the needle hole of the needle also moves from side to side. Accordingly, the hook/upper-thread necessary amount increases in accordance with the swing level of the needle. Conversely, since the motion trajectory of the thread take-up lever is always constant, when the hook/upper-thread necessary amount increases in accordance with the swing of the needle bar, a loosened upper thread cannot be ensured sufficiently, and the upper thread is extraordinary tensioned. This sometimes causes an abnormal noise or a thread breakage.
The present invention has been made to address the above-explained problems of the conventional technologies, and it is an objective of the present invention to provide a sewing machine and a control method thereof which can ensure an upper thread to be sufficiently loosened even if a swing level is increased.